Each Motion Detector will be designed and tested with a specific mounting height. The manual will indicate how high to mount the detector on the wall. Mounting outside of that parameter may cause unexpected behavior.
Make sure the detector is away from air vents, fans, and drafts.
Do not point the motion detector at windows where sunlight might shine in on the lens.
Make sure it is properly angled per instructions if necessary.
Is the sensitivity high?
Often detectors will have a sensitivity setting which allows you to fine tune its operation a bit. Try lowering the sensitivity.
Is the Zone Type correct?
Motion detectors should use the Interior Follower zone type. This is the zone type that only activates during Away arming.
If you have a 2GIG GC3 with firmware 2.2.0 or later, motion detectors must use Interior with Delay zone type instead, due to an unfixed 2GIG bug.
Do you have pets?
Cats are a significant false alarm hazard for motion detectors. This is due to their tendency to climb up to locations where the motion detector’s pet immunity has a harder time filtering them.
Is the sensor pet immune? Some models of motion detector are not designed for use with pets at all.
Try to keep furniture that pets climb on out of range.
Is it a temperature controlled environment?
If the sensor is installed in an outdoor/unregulated environment like a garage, barn, shed, etc., it should be an outdoor rated model. A standard indoor sensor will have a variety of potential false alarm/failure causes.